Interloper is a member of the Eternals.
For unknown reasons he set himself apart from them, having little to do
with them or most of mankind, although he was friends with the Eternal
known as Gilgamesh.
His main reason for being was apparently to fight the evil cosmic being
known as the Dragon of the Moon, who he battled on a number of occasions,
both alone and alongside the Eternals of Earth and of Titan. These battles
took place at undefined times in human history, the most recent one (before
the modern era) occurring during the 6th Century C.E., when
the Interloper and knights of King Arthur's court defeated the Dragon.
Following this battle Interloper apparently lived the life of a hermit,
only emerging in the modern era to fight alongside the heroes the Defenders
against the Dragon of the Moon and then in the service of Doctor
Strange. He currently seems to be bound to the body of a
British man, following his apparent death during a battle with the Dragon
of the Moon.